Mystical field

Imagine a point mass moving through a universe where there are no electric or magnetic fields, and thereby no electric or magnetic forces. There is, however, an unknown "mystical force" that is everywhere. As the point mass moves its mass starts evaporating at a rate of r = 1 g/s r = 1~\mbox{g/s} . However it maintains a constant velocity of V = 5 m/s V = 5~\mbox{m/s} . You observe the point mass at a point A and 10 s 10~\mbox{s} later you observe it at a point B. What magnitude of work in Joules did the mystical force do on the point mass between the points B and A?


The answer is 0.25.

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10 solutions

Jackson T
Aug 5, 2013

Our point mass is losing mass at a constant rate of r 1000 \frac{r}{1000} = x kilograms every second. As the speed is constant it is therefore losing momentum at 5. x every second (the rate of change of momentum). Now as force is the rate of change of momentum it follows our force F is constant (rate of change in momentum is constant) and takes on the above value. F = 5. x . If the force were not to act (we must assume in this universe the laws of physics hold) it's velocity would increase due to the conservation of momentum. So the force is constant and the work done by our force = F.10.V from speed = distance.time and work done = force.displacement . This gives us an answer of 0.25J

Alfredo Saracho
Aug 9, 2013

We know that the Work done (in its simplest form i.e. parallel vectors, all quantities constants) is given by W = F d W = F \cdot d where F F is the force applied to the object, and d d is the distance this object moved. But we also know F = d p d t = d ( m v ) d t = d m d t v + m d v d t F=\frac{dp}{dt}=\frac{d\left(mv\right)}{dt}=\frac{dm}{dt}\cdot v+m\cdot\frac{dv}{dt} Since the velocity is not changing, we have d v d t = 0 \frac{dv}{dt}=0 . We also know that d m d t = 0.001 Kg s \frac{dm}{dt}=0.001\frac{\text{Kg}}{\text{s}} and d = v t = 50 m d=v\cdot t = 50\text{ m} . Substituting into the work expression we get W = 0.25 Joules \boxed{W = 0.25 \text{ Joules}}

I'm not sure why my answer is wrong, can you explain?

Initial kinetic energy = 1 2 m v 2 = 25 2 m 0 \frac{1}{2}mv^{2} = \frac{25}{2}m_{0} .

Final kinetic energy = 25 2 ( m 0 10 1000 ) \frac{25}{2}(m_{0} - \frac{10}{1000})

Change in kinetic energy = 25 2 ( 10 1000 ) = 1 8 J \frac{25}{2}(-\frac{10}{1000}) = -\frac{1}{8} J

By the Work-Energy Principle, Work Done is the same as the change in kinetic energy. So Work Done is 0.125 J -0.125 J .

NB. I'm assuming we discount the work done by the field in removing E = m c 2 E = mc^{2} amount of mass-energy from the object :)

Matt McNabb - 7 years, 10 months ago

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Short Answer: The Work-Energy Principle does not apply to problems with variable masses.

Not so short answer: To derive the Work-Energy Principle, you need the assumption that the force is F = m a F=ma , which everyone told us it was true when we were little. This is ultimately False . In reality, Newton's second law is "The force is equal to the change of rate of the momentum". This is F = d p d t = d ( m v ) d t = d m d t v + m d v d t F=\frac{dp}{dt}=\frac{d\left(mv\right)}{dt}=\frac{dm}{dt}\cdot v + m\cdot \frac{dv }{dt} For most problems, mass does not varies, so d m d t = 0 \frac{dm}{dt}=0 which in turn leads to F = 0 v + m d v d t = m a F=0\cdot v + m \cdot \frac{dv}{dt} = m \cdot a (second law as we know it from kindergarden :) ). A follow up conclusion of this, is the Work-Energy Theorem, which cannot hold if the first term is not zero. The full proof of what I'm saying requires a bit Calculus but it's pretty simple and straightforward. The wikipedia on this subject can guide you through it. Cheers.

Alfredo Saracho - 7 years, 10 months ago

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Is this 'mystery field' even possible? Conservation of energy doesn't apply in it - or, rather, its rules for energy are different to the rules for energy in the real universe. The mass has to go somewhere, and the energy for the work done on the object has to come from somewhere.

Matt McNabb - 7 years, 10 months ago

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@Matt McNabb Yes, it's possible and not even esoteric :p When have you seen in a real life experiment Energy Conservation? It is quite rare.

Have you seen what happens to little pieces of dry ice when you place them over a sheet of water?

Alfredo Saracho - 7 years, 10 months ago

Very clear, thank you :)

Jonathan Wong - 7 years, 10 months ago
Xiaodong Jia
Aug 9, 2013

Use einstein eqn, e=mc^2, but use v instead of c

You had a lucky guess, that's not a valid solution to the problem!

Alfredo Saracho - 7 years, 10 months ago

The force can be expressed as:

F = m d v d t + d m d t v F = m \frac{dv}{dt} + \frac{dm}{dt}v (1)

Since the point mass moves at constant velocity, the first term in (1) is zero:

F = d m d t v = r v F = \frac{dm}{dt}v = rv

Note that since v v is constant, the force is also constant. Work done is expressed as W (in one dimensional motion, with constant force):

W = F x W = Fx (2)

For an object in constant velocity v:

x ( t ) = v t x(t) = vt (3)

Combining all of these:

W = F x = r v x W = Fx = rvx (4)

or

W = r t v 2 = 0.25 W = rt v^2 = 0.25 J (5)

Tilak Patel
Aug 8, 2013

Consider initial mass of the point mass m m

In 10 seconds , mass reduce by 10 gram. As velocity is constant d = 50 m d = 50 m

F = 1 0 2 × 5 10 F = \frac{10^{-2 }\times 5}{10}

F = 5 × 1 0 3 F = 5 \times 10^{-3}

W = w o r k d o n e = F × d W = work done = F \times d

W = 5 × 1 0 3 × 50 W = 5 \times 10^{-3} \times 50

W = 0.25 J W = 0.25 J

Ben Williams
Aug 7, 2013

We know, in general:

( 1 ) F = d p d t (1) F=\frac{dp}{dt}

( 2 ) P = F × v (2) P=F \times v

( 3 ) E = P × T (3) E=P \times T

So, it follows, since

d p d t = 1 0 3 × 5 \frac{dp}{dt}=-10^{-3} \times 5

P = 1 0 3 × 25 P=-10^{-3} \times 25

W = 25 × 1 0 2 = 0.25 J W=-25 \times 10^{-2}=-0.25J

So, W = 0.25 J |W|=0.25J

Caio Andrade
Aug 6, 2013

Momentum variation: dm.V Force=dm.V/t Eenergy=dm.V.d/t=dm.V^2=10E-3x5x5=0.25

Doni Wardoyo
Aug 6, 2013

We can split the rate through dimension : g/s = 10^-3 kg/s. We know that W = F X S, and we know that F = m X a (kg m/s^2). We split this dimension into kg/s X m/s which means "evaporation rate times velocity". If we know this already, then we can do this. Say the evaporation rate is Y, hence we get W = Y X V X V X t = 10^-3 X 5 X 5 X 10 = 0.25

Jian Feng Gao
Aug 6, 2013

Work = Evaporating rate x Velocity^2 x Time

r=1g/s =0.001kg/s

Therefore Work = 0.001(5^2)(10)

=0.25 J

Apart from unit analysis, how is this derived, at all? I'd recommend showing that (Using integration by parts and then by describing the net change from there).

Guillermo Angeris - 7 years, 10 months ago
Erick Sumargo
Aug 5, 2013

Since we know the element " E v a p o r a t i n g R a t e " "Evaporating Rate" , we use a formula :

W = V 2 × r × t W = V^2 \times r \times t

where W = Work (J), V = Velocity ( m s ) \frac{m}{s}) , r = Evaporating Rate ( k g s ) \frac{kg}{s}) and t (s)

By substituting the value, we get W W = 0,25 J.

Is this really a formula, because I just did unit analysis haha

Jian Feng Gao - 7 years, 10 months ago

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That formula is based of from the compatible dimension Juan Feng

Erick Sumargo - 7 years, 10 months ago

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